The present invention generally relates to a liquid crystal display, and more specifically to a multi-domain liquid crystal display having concave structures for achieving a wider viewing angle.
In recent years, liquid crystal displays have become very popular in the market place. High quality liquid crystal displays require high resolution as well as a wide viewing angle. As a result, multi-domain liquid crystal displays have been developed to meet these requirements. In a multi-domain liquid crystal display, each pixel is divided into multiple domains to compensate for the asymmetry in optics to increase the viewing angle of the display.
Conventional multi-domain liquid crystal display technology can be divided into four categories. The first category uses transparent material to form protruded portions or bumps on a substrate to tilt vertically aligned liquid crystals along different directions when an electrical voltage is applied. Although this technique can pre-tilt liquid crystals without a mark rubbing or photo-aligned process, the alignment of liquid crystals is not very stable. It often requires other techniques to stabilize the multi-domain effect.
The second technique relies on slits or openings formed on an ITO electrode layer by etching in cooperation with fringe field effect to form multiple domains similar to the first technique. In practice, the technique can be combined with the first technique to achieve better results. If the second technique is used alone, chiral dopants have to be added to the liquid crystals and the response time is slower. The third technique uses mask rubbing which is a very complicated process with low yield. The fourth technique relies on a photo-aligned method that is still immature.
Forming multiple domains of the conventional multi-domain twisted nematic liquid crystal displays require several rubbing process steps during the manufacturing. Both anti-static charge and prevention of particle contamination are issues that have to be resolved. At present, multiple illumination steps are needed in the technique of the photo-aligned method for the formation of multiple domains.
FIG. 1 shows an example of the conventional multi-domain vertically aligned liquid crystal display with bump structures. The multiple domains in the display are enhanced by means of fringe filed effect and bump structures formed on both upper and lower substrates. As illustrated by the cross-sectional view in FIG. 1, the liquid crystal display 100 comprises a liquid crystal layer sandwiched between two substrates. The lower substrate 108 is a thin film transistor substrate with a pixel electrode layer 105 formed thereon. The upper substrate 109 is a color filter substrate with a common electrode layer 106 formed underneath. A pair of cross polarizers 101 and 102 are attached to the exterior surfaces of the display. Compensation films 103 and 104 are placed between the two polarizers. As can be seen from FIG. 1, a plurality of bump structures 111-117 are formed in the common and pixel electrode layers.
The conventional technique of using transparent material to form bump structures for vertically aligned multi-domain liquid crystal display has another drawback that the bump structures comprise parallel walls which result in disclination lines in the transparent areas of the display. Furthermore, bump structures have to be formed on both upper and lower substrates of the display to ensure that liquid crystals are aligned stably in the multiple domains and avoid the drifting of optical textures. The alignment accuracy, however, may be a problem.
This invention has been made to overcome the above mentioned drawbacks of a conventional multi-domain liquid crystal display. The primary object is to provide a concave structure in each pixel area to form multiple domains. The liquid crystal display of this invention comprises a first substrate, a pasivation layer and a pixel electrode layer formed above the first substrate, a second substrate having a common electrode layer formed underneath, and a liquid crystal layer between the two substrates. A plurality of concave structures are formed in the pixel electrode layer above the passivation layer.
Three other existing techniques are further combined with the concave structure of this invention to provide pre-tilting of liquid crystals in different directions to form multiple domains. These techniques include mask rubbing process, formation of bump structures or openings on an electrode layer in cooperation with fringe field effect. With the multiple domains, the disclination lines are confined in the domain boundary of a pixel area to enhance the contrast.
Accordingly, the structure of the liquid crystal display in this invention requires only one rubbing process or one photo-aligned method for crystal alignment to form multiple domains. The concave structure can be fabricated on the ITO electrode layer by the existing manufacturing process without using additional photo masks. Because the vertically aligned crystals result in a good dark state, the liquid crystal display has very high contrast. In addition, color dispersion is reduced because the liquid crystals are well compensated for the phase difference by the multiple domains.
In one embodiment of this invention, the concave structure is formed in a pixel electrode layer and one or more openings are formed on an electrode layer in a pixel area to combine with fringe field effect for enhancing the multiple domains. In another embodiment, surrounding wall bump structures are formed around the concave structure in a pixel area.
According to the invention, the concave structures can be formed on the pixel electrode layer or the common electrode layer. The openings or bump structures that are used in cooperation with the concave structures may also be formed on one of the electrodes layers. Each concave structure may have a plurality of concave structures. The concave structures may have different shapes.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.